Method of protecting tantalum crucibles against reaction with molten uranium



METHOD OF PROTECTING TANTALUM CRU- ClBLES AGAINST REACTION WITH MOLTEN URANIUM Harold M. Feder, Park Forest, and Norman R. Chellew, Joliet, lll., assignors to theUnited States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Aug. 7, 1957, Ser. No. 676,907

'10 Claims. (Cl. 1486) This invention deals with a method of protecting tantalum metal against attack by molten uranium, with structural units or articles made of tantalum metal thus protected, and also it deals with a method of melting uranium.

In melting uranium, it has always been difiicult to select the proper material for the equipment, in particular the parts with which the molten uranium comes in contact, because most materials that are suitable on account of their mechanical properties and their high melting point react chemically with the molten uranium whereby the uranium is contaminated and the container is corroded. Tantalum metal is one of the materials having the properties just described.

!It has been proposed to use tantalum boride for crucibles or other equipment for melting uranium or holding molten uranium. It was found, however, that the tantalum borides formed by conventional methods did not give the resistance to uranium hoped for and that, when uranium metal was melted in a tantalum boride crucible, for instance, a surface coating was formed where the uranium contacted the crucible and also that the uranium contained an undesirably high amount of boron. A high boron content is a particularly great drawback if the uranium is to be used in nuclear reactors, because boron has a high neutron-capture cross section.

Some of the coatings formed in the tantalum boride crucibles by the contact of molten uranium as just set forth were scraped from the crucible and examined by X' ray; the crucible itself was also subjectedto the same examination. The tests revealed that the tantalum boride of the crucible always corresponded to the formula TaB in which x had a value between 1.78 and 2.56, while the coatings formed in the crucible were found to consistof tantalum monoboride, TaB. From this it was concluded that'the uranium had reacted with tantalum boride of the crucible and reduced it to the monoboride and that at the same time the uranium had taken up the boron released by this reaction. It was also found that, in contradistinc tion to TaB the monoboride TaB was stable and resistant to molten uranium. The process of this invention is based on these findings.

It is an object of this invention to provide a-process of making a tantalum material and articles of tantalum material that are resistant to molten uranium and do not contaminate the molten uranium while in contact therewith.

It is another object of this invention to provide a process of making tantalum material and articles of a tantalum material that do not oxidize in air at temperatures up to about 1800 C.

These objects are accomplished by contacting the tantaice Said boron whereby a boride y ris formed on .the s u r face of said tantalum, removing the boride-coated tantalum from th'e' 'contact with the boron, andheating said borided tantalum at -1800 C. in a vacuum whereby the boride is converted to tantalum monoboride.

The boride formation can becarried out by contacting tantalum with boron powder, boron vapor, or a suspension or slurry of boron powder in a liquid vehicle. If boron powder is used, it is advantageous, though not necessary, to outgas the powder prior to use by heating it at 1000 C. The tantalum article is then packed in the boron powder and heated, while in contact with the boron, to about 1800 C. in a vacuum. The length of heating tie pends on the thickness desired of the protectivelayer.

Deposition of boron from the vapor phase is suitable mainly when a flat member isto be protected on one surface only. In this case the boron powder is placed below the surface to be coated and heated to about 2000 C. in a vacuum. After this, the article is treated like that contacted with boron powder; this will be described later..

If boron application in the form of a slurry is preferred, any liquid vehicle is suitable which is volatileat a comparatively low temperature and does not react with the tantalum or the boron. Various kinds of oil and acetone, for instance, have been found to be satisfactory vehicles. The slurry advantageously contains boron powder in a concentration of between 10 and 30% by weight. The slurry can be applied to the tantalum surface by means known to those skilled in the art. Applying with a paint brush was found suitable. After the slurry has been applied to the surface, the coated article is heated for evaporation of the vehicle; the temperature of this step, of course, depends on the boiling point of the vehicle. .For acetone, room temperature has been found satisfactory for evaporation. After the volatilization step, the boroncoated article is heated to about 1800 C. for boride formation.

' The borided tantalum articles or units are then removed from'contact with the boron. In the case that the boron was applied in the form of a slurry, an excess of any nonlum to be treated with elemental boron, heating the tan- I reacted boron powder is brushed off the surface. There-. after the borided articles are subject to anomer heating step at a, temperature between1800 and,2000-C. whereby y boron present in ss nfthe uaat ty'aecs sa yw form the tantalum monoboride volatilizes and/or diffuses into inner layers of the"tanta'lum"article"until all boride present is in the form of the tantalumfiiionoboride; TaB. In order to ensure complete conversion to the monoboride, the heating should be carried out for .at least fivehours. I V I 7 7 In the following, a number of examples are given for the purpose of illustrating the invention but not with the intention to limit the invention to. the details given therein;

EXAMPLE I Four tantalum coupons 1" x X mils were borided by packing them in boron powder and heating byinduction heat at 1800 C. for the times shown in Table I. Thereafter the coupons were removed from the boron and again heat-treated for the formation of monoboride. The coupons were finally cooled either by shutting off the induction heat, which caused a temperature decrease of 528 C. per minute, or by reducing the induction heat whereby the temperature was loweredby 13 C. per minute.

Table I Boriding Treatment Heat Treatment Weight Increase B dur- Thickness b Specimen No. Weight In- Quench Rate ing heat Surface of Coat Temp, Time, crease dur- Temp., Time, to 980 0. treatment (microns) 0. min. ing boriding 0. min. O./min.) (mg/cm!) (mg/cm?) 1,800 0.62 1,890 297 52s 1. a Smooth--- 47 1,800 5 0. c2 1, 838 300 13 0.00 Smooth. 1,800 10 3. 0a 1, 890 397 52s 0. 99 Crazed.-- 55 1,800 10 2. 63 1, 838 300 1a 0.0 Grimm-.- 05

e Based on total area of coupon. 11 Determined from microscopic examination of section.

The data show that a layer of boride thicker than 50 Table 111 microns cracks easily upon cooling but that the cooling rate is not responsible for this cracking. The consider- Specimen Bedding Heat Phases phases able weight increase which specimens 1 and 3 experienced N umber Treatment Treatment Present Absent during the heat treatment was traced to the fact that o internal parts of the furnace had picked up boron which 1 (1:35 23 6 b T311 subsequently vaporized and deposited on the specimens. 2 1,soo 0., 60 None T iz l iyrao, TaB.

min. thlck-ness offlle found to be 3 1,800 0., 60 1,90o 0.,10 TaB ,TaC, 'IaB fairly well reproducible, and it is mainly dependent on m mm. X3 the time and temperature used in the boriding step. A 4 9- 22 Tan predetermined thickness, however, canbe attained with 5 1,800" 0., 60 1,900 0.,340 TaB,Y TaB g. more precision by brushing the boron onto the tatalum in the form of a slurry.

The next example shows that the quantity of boron applied to the tantalum is not the decisive factor for the quantity of boron reacted with the tantalum and the thickness of the boride layer, but that, as has been stated before, the time and temperature of boriding are the controlling variables.

EXAMPLE II A thin slurry of 20 grams of boron powder (approximate average particle size of 1 micron) in 100 milliliters of petroleum oil of a naphthenic character was painted on tantalum coupons, and the coupons were heated for volatilization of the vehicle first at about 150 C. for 8 hours and then at a temperature of between 150 and 300 C. for another 8 hours. The coupons were then fired as is indicated in Table II.

Table II Time, Boron Boron Temp, O min. Applied, Reacted,

mg. mg.

1 N o excess boron remained.

It will be noticed from this table that at 1800 C. all of the boron had been brought to reaction within 25 minutes. Thus by adjusting the amount of boron applied and heating under these conditions, namely 25 minutes at 1800- C., the thickness of the layer can be controlled.

EXAMPLE III B This composition is the lower limit of a diboride phase which extends from TaBma to TaBMe. R. Kiessling, Acta Chem. Scand., 3, 603 (1949).

b X, Y=unidentified phases.

Table III shows that at least 5 hours of heat treatment are necessary for converting all of the tantalum boride to the monoboride.

=The coupons coated by the above procedure were then heated in contact with uranium droplets at 1200 C. for one hour. After this the droplets were allowed to solidify on the coupons, and both coupon plus uranium were sectioned; the various phases were examined microscopically.

While the uranium phase of specimens 1 through 4 showed many needles of uranium boride, UB the uranium of sample 5 was almost free from such occlusions. All boride layers were cracked which was due to the long boriding time used and the excessive thickness of the boride layer consequently formed.

EXAMPLE IV Molten uranium was held for 8 hours at a temperature of 1350 C. (a) in an uncoated tantalum crucible and (b) in a tantalum crucible borided by the process of this invention. After the 8 hours both crucibles with the uranium in it were cooled, and the entire units were sectioned longitudinally. The section through the nonborided crucible showed that the latter had been heavily attacked; the walls had a nonuniform thickness. and in many places had completely, or almost completely disappeared. The uranium had Wetted the crucible very well which was obvious from the very concave meniscus of the uranium. The borided crucible had retained its uniform wall thickness all over, and the uranium therein had a convex meniscus, which shows that it had a poor wetting action with the borided surface of the crucible.

EXAMPLE V In another instance molten uranium was held in a pressed tantalum boride crucible in which the boride corresponded roughly to the formula TaB The temperature was also 1200 C. and the uranium was held in the crucible at that temperature for 4 hours. The boron content of the uranium in this instance was found to'be 500 p.p.m. This shows clearly the superiority of crucibles coated by the process of this invention.

It will be understood that this invention is not to be limited by the details given herein but that it may be modified within the scope of the appended claims.

What we claim is:

1. A process of protecting the inner surface of a crucible of tantalum metal against the reaction with molten uranium with a well-sealing and well-adhering coating of tantalum monoboride, comprising contacting said inner surface with metallic boron at about 1800 C. in a vacuum whereby a layer of tantalum boride is formed on said surface of said crucible, discontinuing said contact with the boron, and heating said borided crucible to from 1800 to 2000 C. in vacuum whereby the boride is converted to the tantalum monoboride.

2. The process of claim 1 wherein contact with boron is maintained for approximately 5 minutes and heating for the formation of monoboride for atleast 5 hours.

3. The process of claim 1 wherein the temperature in the second heating step ranges between 1800 and 2000 C.

4. The process of claim 1 wherein contact with metallic boron is efiected by packing the tantalum crucible in boron powder.

5. The process of claim 4 wherein the said boron powder had been outgassed by heating to about 1000 C.

6. The process of claim 1 wherein contact with boron is carried out with boron vapor.

7. The process of claim 1 wherein contacting with boron is carried out by applying a slurry of boron powder in a volatilizable nonreacting liquid behicle.

8. The process of claim 7 wherein said slurry contains from 10 to 30% by weight of boron.

9. The process of claim 7 wherein said vehicle is acetone.

10. The process of claim 7 wherein said vehicle is a petroleum oil.

References Cited in the file of this patent UNITED STATES PATENTS 1,968,067 Balke July 31, 1934 1,991,977 Fry Feb. 19, 1935 2,199,125 Barbarou Apr. 30, 1940 2,275,952 Freeman Mar. 10, 1942 2,369,146 Kingston Feb. 13, 1945 2,665,475 Campbell Ian. 12, 1954 2,665,997 Campbell Jan. 12, 1954 2,720,474 Myers Oct. 11, 1955 2,766,110 Meister Oct. 9, 1956 2,789,897 Sawyer Apr. 23, 1957 OTHER REFERENCES AIME Transactions, April 1952, Journal of Metals, page 393. 

1. A PROCESS OF PROTECTING THE INNER SURFACE OF A CRUCIBLE OF TANTALUM METAL AGAINST THE REACTION WITH MOLTEN URANIUM WITH A WELL-SEALING AND WELL-ADHERING COATING OF TANTALUM MONOBORIDE, COMPRISING CONTACTING SAID INNER SURFACE WITH METALLIC BORON AT ABOUT 1800*C. IN A VACUUM WHEREBY A LAYER OF TANTALUM BORIDE IS FORMED ON SAID SURFACE OF SAID CRUCIBLE, DISCONTINUING SAID CONTACT WITH THE BORON, AND HEATING SAID BORIDE CRUCIBLE TO FROM 1800 TO 2000*C. IN VACUUM WHEREBY THE BORIDE IS CONVERTED TO THE TANTALUM MONOBORIDE. 